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Dark Matter, Dark Energy, and the
Fate of the Universe
23.1 Unseen Influences in
the Cosmos
Our goals for learning:
What do we mean by dark matter and dark energy?
What do we mean by dark matter
and dark energy?
Unseen Influences
Dark Matter: An undetected form of mass
that emits little or no light, but whose
existence we infer from its gravitational
influence
Dark Energy: An unknown form of energy that seems to be the source of a repulsive
force causing the expansion of the
universe to accelerate
Contents of Universe
"Ordinary" matter: ~ 4.4%
Ordinary matter inside stars: ~ 0.6%
Ordinary matter outside stars: ~ 3.8%
Dark matter: ~ 23%
Dark energy: ~ 73%
What have we learned?
What do we mean by dark matter and
dark energy?
Dark matter is the name given to the
unseen mass whose gravity governs
the observed motions of stars and gas
clouds.
Dark energy is the name given to
whatever might be causing the
expansion of the universe to
accelerate.
23.2 Evidence for Dark
Matter
Our goals for learning:
What is the evidence for dark matter in
galaxies?
What is the evidence for dark matter in
clusters of galaxies?
Does dark matter really exist?
What might dark matter be made of?
What is the evidence for dark
matter in galaxies?
We measure the mass of the solar system using the orbits of planets:
orbital period
Average distance
For circles:
orbital velocity
orbital radius
Rotation curve
A plot of orbital velocity versus orbital radius
The solar system's rotation curve declines because the Sun has almost all the mass.
Who has the
largest orbital
velocity?
A, B, or C?
Who has the
largest orbital
velocity?
A, B, or C?
Answer: C
The rotation
curve of a
merry-go-round
rises with radius.
The rotation
curve of the
Milky Way stays
flat with
distance.
Mass must be
more spread out
than in the solar
system.
Mass in the Milky
Way is spread out
over a larger
region than its
stars.
Most of the Milky
Way's mass
seems to be dark
matter!
• Mass within the
Sun's orbit:
1.0 1011MSun
Total mass:
~1012MSun
The visible
portion of a
galaxy lies deep
in the heart of a
large halo of
dark matter.
We can
measure the
rotation
curves of
other spiral
galaxies using
the Doppler
shift of the 21-
cm line of
atomic
hydrogen.
• Spiral galaxies all tend to have flat rotation curves,
indicating large amounts of dark matter.
Broadening of
spectral lines in
elliptical galaxies tells
us how fast the stars
are orbiting.
These galaxies also
have dark matter.
Thought Question
What would you conclude about a galaxy
whose rotational velocity rises steadily with
distance beyond the visible part of its disk?
A. Its mass is concentrated at the center.
B. It rotates like the solar system.
C. It's especially rich in dark matter.
D. It's just like the Milky Way.
Thought Question
What would you conclude about a galaxy
whose rotational velocity rises steadily with
distance beyond the visible part of its disk?
A. Its mass is concentrated at the center.
B. It rotates like the solar system.
C. It's especially rich in dark matter.
D. It's just like the Milky Way.
What is the evidence for
dark matter in clusters of
galaxies?
Method #1: Cluster Velocities
We can measure
the velocities of
galaxies in a
cluster from their
Doppler shifts.
Method #1: Cluster Velocities
The mass we find
from galaxy
motions in a
cluster is about
50 times larger
than the mass in
stars!
Clusters contain large amounts of X ray-emitting hot gas.
Temperature of hot gas (particle motions) tells us cluster mass:
85% dark matter
13% hot gas
2% stars
Method #1: Gas
Temperatures
Method #3: Gravitational
Lensing
All three methods of measuring cluster mass
indicate similar amounts of dark matter in galaxy
clusters.
Thought Question
What kind of measurement does not tell us
the mass of a cluster of galaxies?
A. measuring velocities of cluster galaxies
B. measuring the total mass of cluster's stars
C. measuring the temperature of its hot gas
D. measuring distorted images of
background galaxies
Thought Question
What kind of measurement does not tell us
the mass of a cluster of galaxies?
A. measuring velocities of cluster galaxies
B. measuring the total mass of cluster's stars
C. measuring the temperature of its hot gas
D. measuring distorted images of
background galaxies
Does dark matter really
exist?
Our Options
1. Dark matter really exists, and we are
observing the effects of its gravitational
attraction.
2. Something is wrong with our
understanding of gravity, causing us to
mistakenly infer the existence of dark
matter.
Our Options
1. Dark matter really exists, and we are
observing the effects of its gravitational
attraction.
2. Something is wrong with our
understanding of gravity, causing us to
mistakenly infer the existence of dark
matter.
Because gravity is so well tested, most astronomers prefer option #1.
Some observations of the universe are very difficult to explain without dark matter.
What might dark matter
be made of?
Two Basic Options
Ordinary Dark Matter (MACHOs)
Matter made of protons, neutrons,
electrons, but too dark to detect with
current instruments
Extraordinary Dark Matter (WIMPs)
Weakly Interacting Massive Particles:
mysterious neutrino-like particles
Two Basic Options
Ordinary Dark Matter (MACHOs)
Matter made of protons, neutrons,
electrons, but too dark to detect with
current instruments
Extraordinary Dark Matter (WIMPs)
Weakly Interacting Massive Particles:
mysterious neutrino-like particles
The best bet
Measurements of
light element
abundances
indicate that
ordinary matter
cannot account
for all of the dark
matter.
Why Believe in WIMPs?
There's not enough ordinary matter.
WIMPs could be left over from Big Bang.
Models involving WIMPs explain how
galaxy formation works.
What have we learned?
What is the evidence for dark matter in galaxies?
Rotation curves of galaxies are flat, indicating
that most of their matter lies outside their visible
regions.
What is the evidence for dark matter in clusters of
galaxies?
Masses measured from galaxy motions,
temperature of hot gas, and gravitational
lensing all indicate that the vast majority of
matter in clusters is dark.
What have we learned?
Does dark matter really exist?
Either dark matter exists or our understanding
of our gravity must be revised.
What might dark matter be made of?
There does not seem to be enough normal
(baryonic) matter to account for all the dark
matter, so most astronomers suspect that dark
matter is made of (non-baryonic) particles that
have not yet been discovered.